The invention relates to a gas discharge lamp comprising at least one capacitive electrode.
Known gas discharge lamps are composed of a vessel containing a filling gas, wherein the gas discharge takes place, and of generally two metallic electrodes which are sealed in the discharge vessel. An electrode supplies the electrons for the discharge, which electrons are subsequently supplied to the external electric circuit via the second electrode. The donation of electrons generally takes place via thermionic emission (hot electrodes), although it may alternatively be brought about by emission in a strong electric field or, directly, via ion bombardment (ion-induced secondary emission) (cold electrodes). In an inductive mode of operation, the charge carriers are generated directly in the gas volume by means of an electromagnetic alternating field of high frequency (typically above 1 MHz in low-pressure gas discharge lamps). The electrons follow circular paths within the discharge vessel, customary electrodes being absent in this mode of operation. In a capacitive mode of operation, capacitive electrodes are used electrodes. These electrodes are embodied so as to be insulators (dielectric materials), which, on one side, are in contact with the gas discharge and, on the other side, are electroconductively connected (for example by means of a metallic contact) with an external electric circuit. When an alternating voltage is applied to the capacitive electrodes, an electric alternating field is formed in the discharge vessel, and the charge carriers move on the linear electric fields of the alternating field. In the high-frequency range ( greater than 10 MHz), the capacitive lamps are similar to the inductive lamps, because in this range the charge carriers are also generated in the entire gas volume. The surface properties of the dielectric electrode are less important here (so-called xcex1-discharge mode). At lower frequencies, the capacitive lamps change their mode of operation, and the electrons which are important for the discharge must be originally emitted at the surface of the dielectric electrode and multiplied in a so-called cathode drop region to maintain the discharge. Consequently, the emission behavior of the dielectric material determines the functioning of the lamp (so-called xcex3-discharge mode).
A drawback of known gas discharge lamps is that they require drive electronics for their operation. The driver electronics serves to ignite the gas discharge of the lamp and supply a ballast for the operation of the lamp at an electric circuit. Without a suitable lamp ballast in an external electric circuit, the current in the gas discharge lamp would increase so strongly as a result of an increase of charge carriers in the gas volume of the discharge vessel, that the lamp would be rapidly destroyed.
Such gas discharge lamps are also disclosed in U.S. Pat. No. 2,624,858. A gas discharge lamp comprising capacitive electrodes is operated by means of a dielectric material having a high dielectric constant ∈ greater than 100 (preferably ∈ greater than 2000) at an operating frequency below 120 Hz. The external voltage must range between 500 V and 10,000 V. As a result, such a capacitive gas discharge lamp cannot be operated by means of line current for private households (230 V, 50 Hz), but instead requires a circuit comprising drive electronics.
In a gas discharge lamp in accordance with the invention, this object is achieved in that a dielectric is provided having a dielectric saturation polarization P and an effective surface A to form the capacitive electrode, with the product of Pxc2x7A greater than 10xe2x88x925 C. The gas discharge lamp in accordance with the invention comprises a known transparent discharge vessel containing a customary filling gas (for example, for low-pressure gas discharge lamps, an inert gas or an inert gas with mercury). The discharge vessel accommodates at least two spatially separated electrodes, at least one of which is a capacitive electrode. The inventive, capacitive electrode may also be combined with, for example, a metallic electrode. The dielectric of the capacitive electrode may be composed of one or more layers. For each of the dielectric layers, use is made of a material whose dielectric saturation polarization P and effective surface A (i.e. in contact with the plasma in the discharge vessel and with the electric contact) have values such that the product of Pxc2x7A greater than 10xe2x88x925 C. As a result, maximally the electric charge Q=2Pxc2x7A can be transported in one period. In this case, it applies that, on the one hand, the maximum charge Q should be chosen so high that, at an operating frequency f, the mean current Qxc2x7F can flow, and, on the other hand, the lamp is provided with a suitable ballast by the maximum charge. For the dielectric of the capacitive electrode use is preferably made of materials having a saturation polarization P greater than 10xe2x88x925 C/cm2 and an effective surface A of approximately 10 cm2. Naturally, a plurality of further electrodes are conceivable, within the scope of the invention, which suitably combine the material property and geometry of the dielectric material.
Such a lamp can be operated, in particular, using line current for private households (for example 230 V/50 Hz), without a circuit comprising drive electronics.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.